The need for high-precision linear actuators is felt in many areas of technology. For example, mechanical positioning systems such as X-Y tables and robotic assembly systems often require a high degree of precision and control of linearly moving members. Several types of linear actuators have accordingly been developed.
Among the simplest types of linear actuators are those in which the motion of a slide is controlled by the rotation of a worm gear, which engages a suitably threaded element in the slide. One advantage of these actuators is that the position of the slide is easily determined, since the slide is held rigidly in place anytime the worm gear is stopped. A major drawback of these actuators, however, is that rotary torque must be converted into a linear motion. This leads not only to transmission losses and heat, but also to noise, mechanical abrasion, and a short life span for the actuator. Furthermore, such purely mechanical actuation is not able to move the slide at the speeds which are desired in many common applications.
Higher slide speeds and greater efficiency are achieved when the slide is driven electromagnetically. In such systems, a series of electromagnetic coils is normally arranged along the guide rail, with either permanent magnets or electromagnets arranged in the slide. By sequentially activating the electromagnetic coils and changing their polarities relative to the polarities of the slide magnets, the slide is moved along the rail. Known systems of this type have two serious shortcomings.
First, in order to activate the coils sequentially with the proper timing and polarity, some means of commutation is necessary. The most common commutation devices in linear actuators include brushes which slide over slip bars or slip ring tracks. Examples of these systems are described in U.S. Pat. No. 4,644,199 (Langley, Feb. 17, 1987); U.S. Pat. No. 4,560,911 (Chitayat, Dec. 24, 1985); and U.S. Pat. No. 4,733,143 (Chitayat, Mar. 22, 1988). Micro-switches instead of brushes are used in the system described in U.S. Pat. No. 4,439,698 (Chen, Mar. 27, 1984). Not only do brushes create transients and deteriorate due to wear, but even were they to function perfectly, both they and micro-switches cause abrupt, transient-like applications of current to, and shut-offs of, current from the coils they control as the slide moves. These abrupt changes of state in turn cause high-voltage reverse electromagnetic forces, which at best create undesirable signal disturbances and at worst damage other components in the system.
The second major shortcoming of existing coil/magnet actuators arises because of the abrupt activation and shut-off of driving coils. Current surges and high-voltage forces both result in wastes of energy, which enters the system as heat. The heat thus generated increases the thermal deformation of parts of the system such as the guide rail and the slide, and the likelihood of deformation makes it impossible to machine these parts to the fine tolerances which are often desired. For example, the tolerances between the slide and the guide rail in a system using air bearings may be as small as a few ten-thousandths of an inch, so that even slight warping of the rail may cause the slide to seize up. The problem of excess heat is particularly acute in systems which activate all the coils at the same time, such as is done in the system described in the abovementioned Langley patent.
It has been suggested, for example, in U.S. Pat. No. 4,789,815 (Kobayashi, et al., Dec. 6, 1988), to use the properties of Hall-effect elements in linear actuators in order sense and control coil polarity. Even this system, however, relies mostly on current-collecting brushes; no known linear actuator has fully utilized the smooth transition in conductance characteristic of Hall-effect elements.
In order for linear actuators to position a slide or table accurately, some device is also needed for accurately determining the location of the moving member on the guide rail. In addition to purely mechanical position encoders, optical encoders are commonly used. These optical encoders, which often include fragile glass elements, are not suitable for use in harsh environments in which dust, humidity, high temperature, and mechanical shocks are common. Optical encoders are also very sensitive to deformations caused by heat, since the gap between their sensing heads and the scales along which they move is extremely small. Furthermore, in linear actuators using position sensing devices such as brushes and switches, the slide or table often over- or undershoots its intended position, once again because of the mainly binary, impulse-like electrical characteristics of the brushes and switches.